As a wireless access scheme of a digital portable telephone system, a PHS system, etc., a TDMA (Time Division Multiple Access) and TDD (Time Division Duplex) scheme in which TDMA and TDD are combined has been adopted. Additionally, an OFDMA (Orthogonal Frequency Division Multiplexing Access) scheme using OFDMA has been proposed.
The OFDM is a scheme for dividing a carrier to modulate data into a plurality of “subcarriers” (subdivided carriers) orthogonal to each other and distributing and transmitting a data signal in each subcarrier.
Hereinafter, the overview of the OFDM scheme will be described.
FIG. 8 is a block diagram showing a configuration of an OFDM modulation device to be used at a transmitting side. Transmission data is input to the OFDM modulation device. The transmission data is supplied to a serial/parallel conversion unit 201 and converted into data configured from a plurality of low-speed transmission symbols. That is, a plurality of low-speed digital signals are generated by dividing transmission information. Parallel data is supplied to an inverse fast Fourier transform (IFFT) unit 202.
The parallel data is allocated to each subcarrier configuring OFDM and mapped in a frequency domain. Here, each subcarrier is modulated by BPSK, QPSK, 16QAM, 64QAM, etc. The mapping data is transformed from frequency-domain transmission data to time-domain transmission data by performing an IFFT operation. Thereby, multicarrier modulation signals into which a plurality of subcarriers orthogonal to each other are modulated independently are generated. An output of the IFFT unit 202 is supplied to a guard interval adding unit 203.
As shown in FIG. 10, the guard interval adding unit 203 sets a rear part of an effective symbol of transmission data as a guard interval and adds its copy to a front part of an effective symbol period for every transmission symbol. A base-band signal obtained by the guard interval adding unit is supplied to an orthogonal modulation unit 204.
The orthogonal modulation unit 204 orthogonally modulates a base-band OFDM signal supplied from the guard interval adding unit 203 using a carrier signal supplied from a local oscillator 105 of the OFDM modulation device, and performs frequency conversion into an intermediate frequency (IF) signal or a radio frequency (RF) signal. That is, after frequency-converting the base-band signal into a desired transmission frequency band, the orthogonal modulation unit outputs it to a transmission path.
FIG. 9 is a block diagram showing a configuration of an OFDM demodulation device to be used at a receiving side. An OFDM signal generated by the OFDM modulation device of FIG. 8 is input to the OFDM demodulation device through a predetermined transmission path.
An OFDM reception signal input to the OFDM demodulation device is supplied to an orthogonal demodulation unit 211. The orthogonal demodulation unit 211 orthogonally demodulates the OFDM reception signal using a carrier signal supplied from a local oscillator 212 of the OFDM demodulation device, performs frequency conversion from an RF signal or an IF signal to a base-band signal, and obtains a base-band OFDM signal. The OFDM signal is supplied to a guard interval removing unit 213.
The guard interval removing unit 213 removes a signal added by the guard interval adding unit 203 of the OFDM modulation device according to a timing signal supplied from a symbol timing synchronizing unit (not shown). A signal obtained by the guard interval removing unit 213 is supplied to a fast Fourier transform (FFT) unit 214.
The FFT unit 214 performs transformation to frequency-domain reception data by performing an FFT operation on input time-domain reception data. De-mapping is performed in the frequency domain and parallel data is generated for each subcarrier. Here, the demodulation to the modulation of BPSK, QPSK, 16QAM, 64QAM, etc. performed for each subcarrier is performed. Parallel data obtained by the FFT unit 214 is supplied to a parallel/serial conversion unit 215 and output as reception data.
Similar to the above-described OFDM, OFDMA is a method in which a carrier wave is divided into a plurality of sub-carriers, however, it is different from OFDM in that the divided sub-carriers are grouped. Sub-carriers in the group are called “sub-channels,” and a single user occupies these sub-channels or a plurality of users share the sub-channels.
In a wireless transmission path, a signal transmitted from a terminal side propagates through space and is received by a base station side, however, if a plurality of obstacles such as buildings and mountains are present in the transmission path, a reflected wave reflected by an obstacle may reach the base station later than a direct wave. This is called a delay wave, and the delay wave and the direct wave are different in transmission distance from each other, so that the received timings spread due to an influence in the plurality of paths (multipath) on the base station side. The base station sets a timing with the best conditions (for example, the highest transmission power) among these timings as a desired timing. Accordingly, timings other than the desired timing may be distributed before and after the desired timing.
In a communication system adopting a multi-access method in which a plurality of data are simultaneously transmitted from a plurality of terminals to a base station, when the distance to the base station is different among the terminals, the timings at which signals from the terminals arrive at the base station are different. Further, in combination with the above-described influence in the plurality of paths (multipath), the spread of the timings is further increased, and inter-carrier interference may occur.
As a technique for compensating for the inter-carrier interference caused by the above-described propagation delay, Patent Document 1 describes a method in which a temporal alignment control signal is supplied to a plurality of transceivers which transmit signals repeatedly to a common transceiver. Arrival times of signals each of which is transmitted from each of a plurality of transceivers to a common transceiver are detected, and advances and delays of signals which should be transmitted next to the common transceiver are determined so that signals transmitted subsequently from the plurality of transceivers arrive substantially within a predetermined period. To the plurality of transceivers, timing control signals including an advance or delay of the transceivers are transmitted so that the signals transmitted next from substantially all transceivers are advanced or delayed by the amounts determined by the advances and delays included in the timing control signals.
Patent Document 2 describes a method for time synchronization between a part of a plurality of user devices and a head-end device by presumptively calculating a moment at which OFDM symbols are transmitted to a head-end device so that the head-end device can receive the OFDM symbols at a predetermined moment.
Besides the timing control described in the above-described patent document, as a method for eliminating inter-carrier interference, generally, a method in which guard intervals are added is adopted by the terminal side. The guard interval is, as shown in FIG. 10, a copy of the same signal as a part X of the latter half of an effective symbol to the first half of the effective symbol.
On the base station side, by ignoring information on this guard interval, even if only a certain carrier delays, as long as the delay time is within this guard interval, a discontinuous point of the symbols does not occur in sub-carriers after the guard intervals are removed, and even after FFT, interference with an adjacent sub-carrier does not occur, so that the delay is ignored and correct receiving is performed. In the guard interval, data in the effective symbol is copied and inserted, so that even if a certain carrier delays, lack of information does not occur.    Patent Document 1: Japanese Translation of International Application (Kohyo) No. JP-A-2003-528483    Patent Document 2: Japanese Translation of International Application (Kohyo) No. JP-A-2004-533769